1 files changed, 35 insertions, 0 deletions
diff --git a/tests/ui/traits/next-solver/cycles/coinduction/fixpoint-exponential-growth.rs b/tests/ui/traits/next-solver/cycles/coinduction/fixpoint-exponential-growth.rs
new file mode 100644
index 000000000..947b52da7
--- /dev/null
+++ b/tests/ui/traits/next-solver/cycles/coinduction/fixpoint-exponential-growth.rs
@@ -0,0 +1,35 @@
+// compile-flags: -Znext-solver
+
+// Proving `W<?0>: Trait` instantiates `?0` with `(W<?1>, W<?2>)` and then
+// proves `W<?1>: Trait` and `W<?2>: Trait`, resulting in a coinductive cycle.
+//
+// Proving coinductive cycles runs until we reach a fixpoint. However, after
+// computing `try_evaluate_added_goals` in the second fixpoint iteration, the
+// self type already has a depth equal to the number of steps. This results
+// in enormous constraints, causing the canonicalizer to hang without ever
+// reaching the recursion limit. We currently avoid that by erasing the constraints
+// from overflow.
+//
+// This previously caused a hang in the trait solver, see
+// https://github.com/rust-lang/trait-system-refactor-initiative/issues/13.
+
+#![feature(rustc_attrs)]
+
+#[rustc_coinductive]
+trait Trait {}
+
+struct W<T>(T);
+
+impl<T, U> Trait for W<(W<T>, W<U>)>
+where
+    W<T>: Trait,
+    W<U>: Trait,
+{
+}
+
+fn impls<T: Trait>() {}
+
+fn main() {
+    impls::<W<_>>();
+    //~^ ERROR overflow evaluating the requirement
+}